This invention relates generally to a white light illumination system, and specifically to a ceramic YAG:Ce:Gd phosphor for converting blue light emitted by a light emitting diode (xe2x80x9cLEDxe2x80x9d) to white light.
White light emitting LEDs are used as a backlight in liquid crystal displays and as a replacement for small conventional lamps and fluorescent lamps. As discussed in chapter 10.4 of xe2x80x9cThe Blue Laser Diodexe2x80x9d by S. Nakamura et al., pages 216-221 (Springer 1997), incorporated herein by reference, white light LEDs are fabricated by forming a ceramic phosphor layer on the output surface of a blue emitting semiconductor LED. Conventionally, the blue LED is an InGaN single quantum well LED and the phosphor is a cerium doped yttrium aluminum garnet (xe2x80x9cYAGxe2x80x9d), Y3Al5O12:Ce3+. The blue light emitted by the LED excites the phosphor, causing it to emit yellow light. The blue light emitted by the LED is transmitted through the phosphor and is mixed with the yellow light emitted by the phosphor. The viewer perceives the mixture of blue and yellow light as white light.
The chromaticity coordinates of the blue LED, the yellow YAG phosphor and the white combined output of the LED and the phosphor may be plotted on the well known CIE chromaticity diagram, as shown in FIG. 1. The chromaticity coordinates and the CIE chromaticity diagram are explained in detail in several text books, such as pages 98-107 of K. H. Butler, xe2x80x9cFluorescent Lamp Phosphorsxe2x80x9d (The Pennsylvania State University Press 1980) and pages 109-110 of G. Blasse et al., xe2x80x9cLuminescent Materialsxe2x80x9d (Springer-Verlag 1994), both incorporated herein by reference. As shown in FIG. 1, chromaticity coordinates of the prior art blue LEDs used for white emission lie in the circle 1 on the CIE chromaticity diagram in FIG. 1. In other words, the chromaticity coordinates of the LED will be represented by a single point within circle 1, the location of the particular point depending on the peak emission wavelength of the LED.
The chromaticity coordinates of the YAG:Ce3+ phosphor are represented by a point along line 3 in FIG. 1, depending on the level of Gd dopant on the Y lattice site and/or the level of Ga dopant on the Al lattice site. For example, the chromaticity coordinates of the YAG phosphor containing a high level of Gd and/or a low level of Ga dopant may be located at point 5, while the chromaticity coordinates of the YAG phosphor containing a low level of Gd and/or a high level of Ga dopant may be located at point 7. Chromaticity coordinates of the YAG phosphor containing intermediate levels of Gd and/or Ga dopants may be located at any point along line 3 between points 5 and 7, such as at points 9, 11, 13 or 15, for example.
The chromaticity coordinates of the combined output of the blue LED and the YAG phosphor may be varied within a fan shaped region on the CIE chromaticity diagram in FIG. 1, bordered by lines 17 and 19. In other words, the combined chromaticity coordinates of the output of the LED and the phosphor may be any point inside the area bordered by circle 1, line 3, line 17 and line 19 in FIG. 1, as described on page 220 of the Nakamura et al. text book. However, the LEDxe2x80x94phosphor system described by Nakamura et al. suffers from several disadvantages.
As shown in FIG. 1, the CIE chromaticity diagram contains the well known Black Body Locus (xe2x80x9cBBLxe2x80x9d), represented by line 21. The chromaticity coordinates (i.e., color points) that lie along the BBL obey Planck""s equation: E(xcex)=Axcexxe2x88x925/(e(B/T)xe2x88x921), where E is the emission intensity, xcex is the emission wavelength, T the color temperature of the black body and A and B are constants. Various values of the color temperature, T, in degrees Kelvin, are shown on the BBL in FIG. 1. Furthermore, points or color coordinates that lie on or near the BBL yield pleasing white light to a human observer. Typical white light illumination sources are chosen to have chromaticity points on the BBL with color temperatures in the range between 2500K to 7000K. For example, lamps with a point on the BBL with a color temperature of 3900 K are designated xe2x80x9cnatural white,xe2x80x9d a color temperature of 3000 K are designated xe2x80x9cstandard warm white,xe2x80x9d and so on. However, points or color coordinates that lie away from the BBL are less acceptable as a white light to the human observer. Thus, the LEDxe2x80x94phosphor system shown in FIG. 1 contains many points or chromaticity coordinates between lines 17 and 19 that do not yield an acceptable white light for lighting applications.
In order to be useful as a white light source, the chromaticity coordinates LEDxe2x80x94phosphor system must lie on or near to the BBL. The color output of the LEDxe2x80x94phosphor system varies greatly due to frequent, unavoidable, routine deviations from desired parameters (i.e., manufacturing systematic errors) during the production of the phosphor.
For example, the color output of the LEDxe2x80x94phosphor system is very sensitive to the thickness of the phosphor. If the phosphor is too thin, then more than a desired amount of the blue light emitted by the LED will penetrate through the phosphor, and the combined LEDxe2x80x94phosphor system light output will appear bluish, because it is dominated by the output of the LED. In this case, the chromaticity coordinates of the output wavelength of the system will lie close to the LED chromaticity coordinates and away from the BBL on the CIE chromaticity diagram. In contrast, if the phosphor is too thick, then less than a desired amount of the blue LED light will penetrate through the thick phosphor layer. The combined LEDxe2x80x94phosphor system will then appear yellowish, because it is dominated by the yellow output of the phosphor.
Therefore, the thickness of the phosphor is a critical variable affecting the color output of the system. Unfortunately, the thickness of the phosphor is difficult to control during large scale production of the LEDxe2x80x94phosphor system, and variations in phosphor thickness often result in the system output being unsuitable for white light lighting applications or appearing non-white (i.e., bluish or yellowish), which leads to an unacceptably low LEDxe2x80x94phosphor system manufacturing yield.
FIG. 2 illustrates a CIE chromaticity diagram containing the chromaticity coordinates at point 11 of a prior art YAG:Ce3+ phosphor layer that is placed over a blue LED having chromaticity coordinates at point 23. Thus, the chromaticity coordinates of this system will lie along line 25 connecting points 11 and 23 in FIG. 2. If the phosphor layer is thinner than required to produce white light, then too much of the blue LED light will penetrate through the phosphor layer and the chromaticity coordinates of the system light output will lie near the LED coordinates, such as at point 27, below the BBL. The output of this system will appear bluish. If the phosphor layer is thicker than required to produce white light, then too little of the LED light will be absorbed by the phosphor, and the chromaticity coordinates of the system will lie near the phosphor coordinates, such as at point 29, above the BBL. The output of the system will appear yellowish. The chromaticity coordinates of the system will lie near or on the BBL at point 31 only if the thickness of the phosphor layer is almost exactly equal to the thickness required to produce acceptable white light. Thus, FIG. 2 illustrates the sensitivity of the system color output to variations in the phosphor layer thickness.
Furthermore, the prior art LEDxe2x80x94phosphor system suffers from a further deficiency. In order to obtain a white light illumination system with different color temperatures that have color coordinates on or near the BBL (i.e., a system that yields an acceptable white light for illumination purposes), the composition of the phosphor has to be changed. For example, if a prior art system includes a phosphor having a composition whose color coordinates are located at point 11 in FIG. 2, then the LEDxe2x80x94phosphor system containing this particular phosphor will have color coordinates near the BBL (i.e., near point 31 on line 27) only for a narrow color temperature range between about 5800 K and 6800 K. The system with this particular phosphor composition does not yield an acceptable white light for lighting applications for color temperatures outside this range. Therefore, the phosphor composition must be changed in order to obtain a system which yields an acceptable white light for lighting applications for desired color temperatures outside the range of 5800 K to 6800 K. The required change in the phosphor composition increases the cost and complexity of the manufacturing process. The present invention is directed to overcoming or at least reducing the problems set forth above.
In accordance with one aspect of the present invention, there is provided a white light illumination system comprising a radiation source and a luminescent material, wherein an emission spectrum of the radiation source represents a first point on a CIE chromaticity diagram, an emission spectrum of the luminescent material represents a second point on the CIE chromaticity diagram and a first line connecting the first point and the second point approximates a Black Body Locus on the CIE chromaticity diagram.
In accordance with another aspect of the present invention, there is provided a white light illumination system, comprising a luminescent material, comprising (A1xe2x88x92xGdx)3D5E12:Ce, wherein A comprises at least one of Y, Lu, Sm and La, D comprises at least one of Al, Ga, Sc and In, E comprises oxygen, x greater than 0.4, and a light emitting diode having a peak emission wavelength greater than 470 nm.
In accordance with another aspect of the present invention, there is provided a method of making a white light illumination system containing a radiation source and a luminescent material, comprising selecting a first line which approximates a Black Body Locus on a CIE chromaticity diagram, forming the radiation source, wherein an emission spectrum of the radiation source is represented by a first point on the first line and forming the luminescent material, wherein an emission spectrum of the luminescent material is represented by a second point on the first line.